The subject invention is directed, generally, to methods and systems suitable for use in photolithography and, more particularly, to methods and systems suitable for use in photolithography employing ArF excimer lasers.
Projection optical lithography systems have been used for some time now in the manufacture of integrated circuits. Recently, driven in part by the desire to achieve smaller and smaller features, optical lithography systems used by the semiconductor industry in the manufacture of integrated circuits have progressed towards shorter wavelengths of light, such as the popular 248 nm and 193 nm wavelengths. Such systems benefit greatly from the use of refractive optics made from materials having high transmittance. High purity fused silica exhibits the desired transmittance and, consequently, has become a widely-used material for making the refractive optics found in 248 nm and 193 nm photolithographic systems. In addition, high purity fused silica exhibits excellent chemical durability and dimensional stability, and these properties have also made high purity fused silica well suited for use as optical lenses and other optical components in photolithographic systems.
The behavior of high purity fused silica for 248 and 193 nm laser-based photolithography has been extensively studied. In particular, these studies have included investigations into laser-induced xe2x80x9cdamagexe2x80x9d, both damage due to induced absorption and damage due to induced density changes. In general, these studies have been carried out at a relatively high exposure fluence in order to accelerate the test. For example, rather than performing the test for a period of time T using an exposure fluence of F, the test would be performed for a period of time T/x using an exposure fluence of xF, on the theory that the aggregate amount of light to which the sample is exposed would be the same in either case. Using these accelerated tests, all silica, irrespective of the supplier, exhibit positive induced density changes, a phenomenon commonly referred to as xe2x80x9cdensificationxe2x80x9d or xe2x80x9ccompactionxe2x80x9d. Furthermore, again using these accelerated tests, the densification behavior has been quantitatively described over a wide range of exposures by a power-law expression having the following form (xe2x80x9cEquation 1xe2x80x9d):       Δρ    ρ    =            α      ⁡              (                              NF            2                    τ                )              b  
where xcex94xcfx81/xcfx81 represents the relative density change, F is the exposure fluence, N is the number of pulses, xcfx84 is a measure of the pulse duration, and b and xcex1 are constants which may vary from wavelength to wavelength but do not vary from glass to glass. Thus, it has generally been believed that a high purity silica glass will experience a laser-induced change in its index of refraction, but that this change evolves in a predictable way (e.g., as described by Equation 1) so that some sort of programmed correction can be applied (e.g., by adjusting the positions and/or orientation of lenses or other optical components).
To further understand the behavior of high purity silica glasses in laser-based photolithography systems, tests were recently conducted at the exposure fluences more appropriate to those which are typically employed in actual laser-based photolithography systems. The results showed that high purity silica glasses behave differently, depending on the supplier of the silica sample. For example, in certain samples, xe2x80x9cexpansionxe2x80x9d (i.e., decreased density), not desification, was observed after exposure to laser radiation. These tests and results are described in Van Peski et al., J. Non-Cryst. Sol., 265:285 (2000) (xe2x80x9cVan Peskixe2x80x9d), which is hereby incorporated by reference.
Applicants have further studied the effects of pulsed ultraviolet radiation exposure on high purity silica glasses using two methods: birefringence and interferometry. Each of these methods measures a different aspect of the same induced volume change. The former measures birefringence which results from the stresses that are produced by volume changes (e.g., densification or expansion), whereas interferometry measures changes in the refractive index associated with the volume change caused by densification or expansion. In the high fluence work cited above (i.e., in the accelerated tests), estimates of the volume change as measured by the two techniques have agreed within experimental error.
Applicants have found that, when the dissolved molecular hydrogen concentration in high purity fused silica is above a certain level (e.g., above 0.5xc3x971018 molecules H2/cm3 SiO2) and when the fluence is low (e.g., below roughly 10 mJ/cm2/pulse), changes in the high purity fused silica""s refractive index resulting from exposure to pulsed ultraviolet radiation cannot be fully explained in terms of densification, such as predicted by Equation 1 or as described in, e.g., Borrelli et. al, J. Opt. Soc. Am. B, 14(7):1606 (1997), which is hereby incorporated by reference.
More particularly, applicants have found that there are two additional effects concurrent with the expected densification when silica with high molecular hydrogen content is exposed to low fluence ultraviolet radiation. They are expansion and photorefraction. As used herein, xe2x80x9cphotorefractionxe2x80x9d is meant to refer to a refractive index increase that occurs without any volume change. Furthermore, applicants have observed that the magnitude of both of these effects is strongly dependent on the fluence and the molecular hydrogen concentration. Moreover, because the photorefraction effect has no stress associated with it, birefringence measurements do not give the same result as interferometry for high purity fused silica having high molecular hydrogen concentration exposed to relatively low fluence. In general, the laser damage specification is in terms of wavefront distortion, which in turn is strongly dependent on changes in refractive index. Since interferometry measures refractive index directly, it is the more appropriate measurement. On the other hand, if birefringence is used to estimate the refractive index change, it will only see changes originating from the volume changes. This, coupled with the fact that prior investigations into laser-induced damage have used accelerated tests (e.g., as described above, using relatively high exposure fluences), has resulted in an inaccurate understanding of the factors which should be taken into account with regard to molecular hydrogen concentration when designing or selecting high purity silica glasses for use in ultraviolet photolithography and other methods which employ pulsed ultraviolet radiation. Accordingly, a need continues to exist for new ultraviolet photolithography methods and systems.
The present invention relates to a lithography method. A pulsed ultraviolet radiation source for producing ultraviolet lithography radiation having a wavelength shorter than about 300 nm at a fluence of less than 10 mJ/cm2/pulse and a high purity fused silica lithography glass having a concentration of molecular hydrogen of between about 0.02xc3x971018 molecules/cm3 and about 0.18xc3x971018 molecules/cm3 are provided. A lithography pattern is formed with the ultraviolet lithography radiation. The lithography pattern is reduced to produce a reduced lithography pattern, and the reduced lithography pattern is projected onto a ultraviolet radiation sensitive lithography medium to form a printed lithography pattern. At least one of the forming, reducing, and projecting steps includes transmitting the ultraviolet lithography radiation through the high purity fused silica lithography glass.
The present invention relates to another lithography method. In this method, a pulsed ultraviolet radiation source for producing ultraviolet lithography radiation having a wavelength shorter than about 300 nm at a fluence of less than 10 mJ/cm2/pulse and a high purity fused silica lithography glass having a concentration of molecular hydrogen of between about 0.05xc3x971018 molecules/cm3 and 0.18xc3x971018 molecules/cm3 or having a concentration of molecular hydrogen of between 0.22xc3x971018 molecules/cm3 and about 0.5xc3x971018 molecules/cm3 are provided. A lithography pattern is formed with the ultraviolet lithography radiation. The lithography pattern is reduced to produce a reduced lithography pattern, and the reduced lithography pattern is projected onto a ultraviolet radiation sensitive lithography medium to form a printed lithography pattern. At least one of the forming, reducing, and projecting steps includes transmitting the ultraviolet lithography radiation through the high purity fused silica lithography glass.
The present invention also relates to lithography systems which include a pulsed ultraviolet radiation source for producing ultraviolet lithography radiation having a wavelength shorter than about 300 nm at a fluence of less than 10 mJ/cm2/pulse. The lithography systems also include at least one synthetic glass optical member which transmits lithography radiation from the pulsed ultraviolet radiation source. In one inventive lithography system, the at least one synthetic glass optical member includes a high purity fused silica lithography glass having a concentration of molecular hydrogen of between about 0.02xc3x971018 molecules/cm3 and about 0.18xc3x971018 molecules/cm3. In another inventive lithography system, the at least one synthetic glass optical member includes a high purity fused silica lithography glass having a concentration of molecular hydrogen of between about 0.05xc3x971018 molecules/cm3 and 0.18xc3x971018 molecules/cm3 or having a concentration of molecular hydrogen of between 0.22xc3x971018 molecules/cm3 and about 0.5xc3x971018 molecules/cm3.
The present invention also relates to a method for producing a synthetic high purity fused silica glass optical member having a predictably evolving wavefront distortion when exposed to pulsed ultraviolet lithography radiation having a wavelength shorter than about 300 nm at a fluence of less than 10 mJ/cm2/pulse. The method includes limiting molecular hydrogen concentration in the high purity fused silica glass optical member to between about 0.05xc3x971018 molecules/cm3 and about 0.5xc3x971018 molecules/cm3.
The present invention also relates to synthetic glass optical members for use with pulsed ultraviolet radiation having a wavelength shorter than about 200 nm and a fluence of less than about 8 mJ/cm2/pulse. In one inventive synthetic glass optical member, the member includes a high purity fused silica glass having a concentration of molecular hydrogen of between about 0.05xc3x971018 molecules/cm3 and about 0.18xc3x971018 molecules/cm3. In another inventive synthetic glass optical member, the member includes a high purity fused silica glass having a concentration of molecular hydrogen of between about 0.05xc3x971018 molecules/cm3 and 0.18xc3x971018 molecules/cm3 or having a concentration of molecular hydrogen of between 0.22xc3x971018 molecules/cm3 and about 0.5xc3x971018 molecules/cm3. In still another inventive synthetic glass optical member, the member includes high purity fused silica glass having a concentration of molecular hydrogen sufficiently low so that wavefront distortion caused by the high purity fused silica glass evolves predictably over time.